Why does my aux cable allow podcasts to play through my car speakers but not phone conversations?

[AC Question]

I have a moto g4 and when I plug the aux cable into it podcasts play through the car speakers fine but if I make a call the audio comes through the phone itself until the call ends and then all audio (including button pressing sounds) come back through the car speakers again.

With my old andriod phone (archos) this wasn't an issue at all so I'm hoping its a setting somewhere?

Any help would be awesome!